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Abstract:

A device (1) for adding reverberation to an input signal (s(n)) uses a
corresponding transformed input signal (S(n)), for example the Fourier
transform of the signal, to generate reverberation. The device comprises
circuits (3, 5, 6) for combining the transformed input signal (S(n)) with
a modified and delayed transformed output signal (G(Sr(n-i))) so as to
produce a current transformed output signal (Sr(n)). Inversely
transforming the current transformed output signal (Sr(n)) results in an
output signal (sr(n)) having reverberation.

Claims:

1. A device for adding reverberation to an input signal represented by a
corresponding transformed input signal, the device comprising:delay means
for delaying the transformed output signal so as to produce a delayed
transformed output signal, andcombination means for combining the
transformed input signal with a delayed transformed output signal so as
to produce a current transformed output signal representing an output
signal having reverberation.

2. The device of claim 1, further comprising modification means for
modifying the amplitude and/or the phase of the delayed transformed
output signal so as to produce a modified and delayed transformed output
signal.

3. The device of claim 2, wherein the modification means are arranged for
providing a substantially random phase.

4. The device of claim 3, wherein the random phase is bound by a maximum
phase differences, said maximum phase difference being substantially
smaller than a threshold for low frequencies and substantially equal to
said threshold for high frequencies.

5. The device of claim 3, wherein the modification means are arranged for
an amplitude adjustment which involves a relatively small attenuation for
low frequencies and a relatively large attenuation for higher
frequencies.

6. The device of claim 1, comprising multiple parallel branches, each
branch being provided with processing means for combining a partial
transformed input signal with a modified and delayed transformed partial
output signal so as to produce a current transformed partial output
signal, the device further comprising at least one further combination
unit for combining the transformed partial output signals into a
transformed combined output signal.

7. The device of claim 6, wherein at least one branch comprises at least
one scaling unit for scaling at least one partial output signal.

8. The device of claim 1, wherein the processing means are arranged for
processing the transformed input signal per frame.

9. The device of claim 1, further comprising a transform unit for
transforming an input signal into a transformed input signal.

10. An audio system comprising a device claim 1.

11. A method of adding reverberation to an input signal represented by a
corresponding transformed input signal, the method comprising:delaying
the transformed output signal so as to produce a delayed transformed
output signal, andcombining the transformed input signal with a delayed
transformed output signal so as to produce a current transformed output
signal representing an output signal having reverberation.

12. A computer program product for carrying out the method claim 11.

Description:

FIELD OF THE INVENTION

[0001]The present invention relates to a device for and a method of adding
reverberation to signals, in particular audio signals.

BACKGROUND OF THE INVENTION

[0002]It is well known to add reverberation to audio signals to imitate
the acoustic effects of a room, or of a concert hall. Reverberation may
be generated electronically using a so-called delay line, an element
which delays the audio signal over a certain time period by temporarily
storing the signal samples in successive memory elements. However, to
obtain a natural-sounding effect, very large delay lines are required. A
typical reverberation time of 2 seconds requires a delay element that is
50,000 to 100,000 samples long, a size that is difficult and uneconomic
to implement.

[0003]Various attempts have been made to overcome this problem. U.S. Pat.
No. 5,917,917 (Jenkins), for example, suggests decimating the audio
signal prior to applying the signal to a reverberation device, and
subsequently interpolating the resulting signal to restore the sampling
frequency. It will be clear that this approach is likely to introduce
artifacts into the signal while still requiring a large delay line.

[0004]Alternative solutions suggest calculating the convolution of the
audio signal and the impulse response of a room or concert hall to obtain
reverberation. However, such impulse responses are typically long and the
computational effort involved in such solutions is very large, while
large memories are still necessary.

SUMMARY OF THE INVENTION

[0005]It is an object of the present invention to overcome these and other
problems of prior art and to provide a device and a method of adding
reverberation to a signal, which device and method require substantially
less memory and less computational effort.

[0006]Accordingly, the present invention provides a device for adding
reverberation to an input signal represented by a corresponding
transformed input signal, the device comprising delay means for delaying
the transformed output signal so as to produce a delayed transformed
output signal, and combination means for combining the transformed input
signal with a delayed transformed output signal so as to produce a
current transformed output signal representing an output signal having
reverberation.

[0007]By using transformed signals instead of time signals, a very
realistic reverberation effect can be obtained while avoiding the memory
requirements and computational cost involved in calculating convolutions.
Transforming the input signal can be carried out using standard optimized
transformations, such as the Fast Fourier Transform (FFT).

[0008]In the present invention, the transformed output signal is based
upon both the transformed input signal and a delayed version of the
previous output signal. This delayed version of the previous output
signal is preferably modified in amplitude and/or phase so as to be able
to provide the desired reverberation.

[0009]The delay means are arranged for temporarily storing the output
signal so as to produce the previous output signal. Typically, the delay
means will store the signal for the duration of one frame, in which case
the term "previous" is understood to mean, "immediately preceding".
However, the invention is not so limited and the delay means may also
store the signal for the duration of two or more frames, in which case
the term "previous" is understood to mean "existing before in time or
order". The delay means may be constituted by means of a memory unit, a
suitable register, or other delay unit.

[0010]It is noted that a frame is typically understood to comprise a set
of signal samples, for instance 256, 512 or 1024 samples, which are
together subjected to a transformation such as the Fast Fourier Transform
(FFT). Depending on the sampling frequency, a frame may represent certain
time duration. At a sampling speed of 44.1 kHz, for example, a frame of
1024 samples has duration of approximately 23.2 ms.

[0011]In a preferred embodiment, the device of the present invention
comprises modification means for modifying the amplitude and/or the phase
of the delayed transformed output signal so as to produce a modified and
delayed transformed output signal. The delayed transformed output signal,
which is used to produce a new output signal, is preferably modified
before being combined with the (new) transformed input signal. This
modification may comprise an amplitude modification by applying a gain
(or attenuation), but more importantly comprises a phase modification.

[0012]In particular, the present invention proposes to modify the phase of
the transformed delayed output signal such that its phase changes
randomly or pseudo-randomly. This substantially random phase modification
avoids repetitive patterns in the reverberation. Such patterns would give
rise to undesirable audible artifacts.

[0013]In a further advantageous embodiment, the random phase modification
is bound by a maximum phase difference, said maximum phase difference
being substantially smaller than p for low frequencies and substantially
equal to p for high frequencies. That is, a frequency-dependent limit is
imposed on the phase modification imparted by the modification means so
as to avoid undesirable effects. While the random phase may fluctuate
between -p and p rad for high frequencies (in a typical but non-limiting
example, for frequencies higher than approximately 300 Hz), the random
phase which may be added to the original phase is very small for low
frequencies (in a typical but non-limiting example, for frequencies lower
than approximately 100 Hz) in order not to distort low-frequency signal
components.

[0014]As mentioned above, the modification unit may also apply a gain (or,
if the gain is smaller than one, an attenuation) to the delayed
transformed output signal in order to control the amplitude of the
reverberation and, consequently, the reverberation time. In a preferred
embodiment, the modification means are arranged for an amplitude
modification that involves a relatively small attenuation for low
frequencies and a relatively large attenuation for higher frequencies.
The frequency-dependent attenuation allows a reverberation time, which is
longer at lower frequencies and gradually decreases for higher
frequencies. This corresponds with the reverberant behavior of typical
rooms.

[0015]It is further preferred that the distinction between "low" and
"high" frequencies is similar to that used for the phase. Using the above
example, the attenuation may be relatively small for frequencies up to
approximately 100 Hz and relatively high for frequencies from
approximately 300 Hz. It will be understood that these values are
exemplary only and that other values may be used, such as 1000 and 3000
Hz, without departing from the present invention.

[0016]In the above discussion it has been assumed that the device of the
present invention receives a single input signal. The invention, however,
is not so limited and also provides a device that is capable of receiving
multiple input signals. In a particularly advantageous embodiment the
device of the present invention comprises multiple parallel branches,
each branch being provided with processing means for combining a partial
transformed input signal with a modified and delayed transformed partial
output signal so as to produce a current transformed partial output
signal, the device further comprising at least one further combination
unit for combining the transformed partial output signals into a
transformed combined output signal. The partial transformed input signals
may, for example, represent individual instruments in a musical
instrument digital interface (MIDI) sound bank, or may each represent a
sum of various instruments in a MIDI sound bank. The partial transformed
input signals may be stored directly in a MIDI sound bank or may be
synthesized in the frequency domain from a parametric representation of
the audio signals in a MIDI sound bank.

[0017]Advantageously, at least one branch may comprise at least one
scaling unit. The scaling unit allows the contribution from the branch to
be controlled. In some embodiments, a branch may contain two scaling
units, one for scaling the delayed transformed output signal, and one for
scaling the transformed output signal itself. Advantageously, a scaling
unit is incorporated in the modification means of each branch.

[0018]It is preferred that the processing means are arranged for
processing the transformed input signal per frame. This allows the
processing means to process a plurality of samples substantially
simultaneously, thus increasing the processing speed. In addition, any
transform unit typically operates on a set of samples simultaneously and
using frames therefore facilitates this processing. It will be understood
that the actual frame size used is not relevant, although sizes that are
powers of two (such as 512 and 1024) are of course convenient in digital
signal processing.

[0019]The at least one input signal may be received as a transformed input
signal produced by an external transform unit or by a storage device on
which transformed signals are stored. However, when the input signal is
received as a time signal, the device of the present invention
conveniently further comprises a transform unit for transforming an input
signal into a transformed input signal. In case multiple time signals are
received, multiple transform units may be provided. In addition, at least
one A/D (analog/digital) converter may be provided in case the input
signal is an analog signal. The device of the present invention may
advantageously also comprise an inverse transform unit for transforming
the current transformed output signal into a time signal.

[0020]Although the device of the present invention may be made up of
distinct units, such as a combination unit and a delay (memory) unit, it
is also possible for the device to be constituted by a microprocessor or
a microcomputer executing suitable software instructions.

[0021]The present invention can also be said to provide a device for
adding reverberation to an audio signal represented by its frequency
spectrum, the device comprising delay means for delaying the frequency
spectrum so as to produce a delayed frequency spectrum, modification
means for modifying the phase of the delayed frequency spectrum so as to
produce a phase-adjusted frequency spectrum, combination means for
combining the phase-adjusted frequency spectrum with the original
frequency spectrum so as to produce a combined frequency spectrum, and
inverse transform means for inversely transforming the combined frequency
spectrum so as to produce an audio signal having reverberation, wherein
the modification means are arranged for providing a substantially random
phase. The modification means may also be arranged for modifying the
amplitude of the delayed frequency spectrum.

[0022]The present invention further provides an audio system comprising a
device as defined above. Said audio system may be an electronic musical
instrument, such as an electronic organ, a keyboard or a synthesizer,
comprising a device as defined above, as well as a ring tone synthesizer,
in particular for use in a mobile telephone or a gaming device,
comprising a device according to the present invention.

[0023]The present invention additionally provides a method of adding
reverberation to an input signal represented by a corresponding
transformed input signal, the method comprising the steps of delaying the
transformed output signal so as to produce a delayed transformed output
signal, and combining the transformed input signal with a delayed
transformed output signal so as to produce a current transformed output
signal representing an output signal having reverberation.

[0024]The aspects defined above and further aspects of the invention are
apparent from the examples of embodiment to be described hereinafter and
are explained with reference to these examples of embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]The present invention will further be explained below with reference
to exemplary embodiments illustrated in the accompanying drawings, to
which the invention is not limited.

[0026]FIG. 1 schematically shows a first embodiment of a device according
to the present invention.

[0027]FIG. 2 schematically shows a second embodiment of a device according
to the present invention.

[0028]FIG. 3 schematically shows an audio system, which incorporates a
device according to the present invention.

[0029]FIG. 4 schematically shows an exemplary gain adjustment as used in
the present invention.

[0030]FIG. 5 schematically shows an exemplary maximum phase adjustment as
used in the present invention.

[0032]In the present example, the transform unit 2 receives a time signal
s(n). This time signal s(n) is digital (or digitized), with n indicating
the frame number and each frame containing a number of samples. Those
skilled in the art will understand that an A/D (Analog/Digital) converter
should be provided in case an analog input signal is received. The time
signal s(n) typically is an audio signal to which reverberation are to be
added.

[0033]The transform unit 2 receives the time signal s(n) and outputs a
transformed signal S(n). Typically, the transform unit 2 applies a Fast
Fourier Transform (FFT), in which case the transformed signal S(n) is the
frequency spectrum of the time signal s(n), with n again indicating the
frame number and the frame now containing a number of frequency
components.

[0034]Instead of the (Fast) Fourier Transform, other transforms, such as
the (modified) cosine transform, may also be applied. It is further
possible for the device 1 to receive the frequency spectrum S(n) instead
of the time signal s(n), or to directly synthesize the frequency spectrum
S(n) in the frequency domain from a parametric representation of the
signal s(n), in which cases the transform unit 2 may be omitted. The
transformed signal S(n) is fed to a combination unit 3, which in the
embodiment shown is constituted by a signal adding circuit.

[0035]In the combination circuit 3, the transformed signal S(n) is
combined with (that is, added to) a delayed and modified version of the
transformed output signal Sr(n) to produce a new transformed output
signal:

Sr(n)=S(n)+G(Sr(n-i)) (1)

where i indicates the amount of delay introduced by the memory (M) 5 and G
indicates the gain and/or phase adjustment provided by the modification
unit 6. The amount of delay is, in the present example, expressed in
frames. A typical delay is equal to one frame (i=1), although delays of
two, three or more frames are also possible, depending of the time
duration of each frame and the desired reverberation or reverberation
time. The gain and the phase adjustment, if any, may vary in time.

[0036]The memory unit 5, which serves as a delay unit, temporarily stores
one or more frames of the transformed output signal Sr(n) to produce a
signal Sr(n-i). A delayed frame is output to the modification unit 6
which modifies the phase and possibly also the gain of the transformed
signal Sr(n-i) to provide a phase and/or gain adjusted transformed signal
G(Sr(n-i)) which is fed to the combination unit 3, as discussed above.

[0037]It is noted that the present invention does not require the use of
an impulse response or its Fourier transform for producing reverberation.
Instead, the present invention produces reverberation using the delayed
transform (for example the Fourier transform) of the signal itself.

[0038]In a preferred embodiment, the modification unit 6 applies a random
or pseudo-random phase shift relative to the original phase. This has the
advantage of decorrelating the signals G(Sr(n-i)) and S(n), which are
combined in the combination unit 3. If these signals were correlated,
repetitive patterns would occur in the reverberation.

[0040]It can thus be seen that the device 1 produces reverberation in a
very simple and effective manner. By producing reverberation in the
transformed (typically: frequency) domain instead of in the time domain,
the use of long delay lines is avoided.

[0041]The device 1 of FIG. 1 provides uniform reverberation for the entire
signal s(n). It is also possible to provide individual reverberation for
separate frequency bands or instruments. Using MDI-technology, individual
instruments may be processed separately. An exemplary embodiment of the
device 1 of the present invention that allows such separate processing is
schematically illustrated in FIG. 2.

[0042]In the embodiment of FIG. 2, individual transformed signals (for
example frequency spectra) S1(n), S2(n), . . . Sm(n) are received and fed
to a respective combination unit 3. These transformed signals may
originate from a MIDI data storage, or from a filter bank to which an
original (composite) transformed signal S(n) or time signal s(n) is fed.
These transformed input signals may be retrieved from a MIDI sound bank
or may be synthesized in the frequency domain from a parametric
representation of the audio signals in a MIDI sound bank.

[0043]The device of FIG. 2 has m parallel branches which each comprise a
combination unit 3, a memory unit 5, a gain/phase modification unit 6,
and a scaling unit 7. Each branch (except the mth one) also comprises a
further combination unit 9 for combining the output signal of the branch
concerned with that of the other branches. An inverse transform unit 4
produces a time (output) signal sr(n) containing reverberation, as in the
embodiment of FIG. 1.

[0044]The advantage of the embodiment of FIG. 2 is that the reverberation
can be controlled per instrument or per channel. More in particular, a
scaling may be provided by means of the modification units 6, which are
preferably arranged for modifying both the phase and the amplitude of the
delayed transformed output signals. Each modification unit 6 may have an
individual gain G1, G2 . . . , Gm which individual gain serves as a
scaling factor and controls the reverberation in the respective branch.
The individual gains G1, G2 . . . Gm may be pre-set, user controlled, or
extracted from MIDI content.

[0045]Further scaling may be provided by means of scaling units 7. Said
scaling units 7 may be provided in each branch or in (m-1) branches.
Scaling units 7 each receive a scaling factor aj (j=1 . . . m) and
multiply the signal Sr(n) by this scaling factor so as to produce a
scaled signal aj .Sr(n) which is fed to the respective further
combination unit 9. The scaling factors aj determine the relative
contribution of each branch to the combined transformed output signal
Sr(n).

[0046]The scaling units 7 may be integrated in the further combination
units 9 so as to form weighted combination units. Additionally, or
alternatively, the individual further combination units 9 shown in FIG. 2
may be combined into a single further combination unit having multiple
inputs.

[0047]The scaling factors aj may also be pre-set, user controlled, or
extracted from MIDI content. Embodiments can be envisaged in which
scaling units are arranged before the combination units 3 so as to scale
the transformed input signals.

[0048]In the embodiment of FIG. 2 it has been assumed that the input
signals sj(n) are available as transformed signals Sj(n). If this is not
the case, suitable transform units (2 in FIG. 1) can be provided.

[0049]The operation of the gain/phase modification units 6 will be further
explained with reference to FIGS. 4 and 5. An exemplary gain A(G) is
illustrated in FIG. 4. In this particular example, the gain A(G) is
smaller than one and therefore constitutes attenuation (Att), measured in
decibels per second (dB/s), as a function of the frequency (f), measured
in Hertz (Hz). In the example of FIG. 4, the attenuation is shown to be
approximately 60 dB/s for frequencies ranging from 0 to about 200 Hz, and
approximately 120 dB/s for frequencies above about 300 Hz. Thus a
frequency-dependent gain (that is, attenuation) is provided which
controls the reverberation time as a function of the frequency.

[0050]The graph of FIG. 5 shows an exemplary maximum phase difference
(Maximum Different Phase, MDP), expressed in rad, as a function of the
frequency (f), measured in Hertz (Hz). As mentioned above, the gain/phase
modification unit 6 produces a substantially random phase. However, in
order to avoid signal distortion, the phase is preferably constrained.
That is, the phase difference introduced by the gain/phase modification
unit 6 is preferably limited to the value Φ(G) shown in FIG. 5. In
the example shown, this value is close to zero for frequencies up to
about 100 Hz, then rises steeply to a level of p rad, and subsequently
maintains this level. Limiting the phase difference introduced by the
gain/phase modification unit 6 avoids signal distortion.

[0051]An audio system 10 comprising at least one device 1, 1' of the
present invention is schematically illustrated in FIG. 3. The audio
system 10 further comprises a windowing (W) unit 11 for applying a time
window to a digital (or digitized) signal s(k), where k indicates the
sample number, so as to obtain a signal s(n) comprised of frames, where n
indicates the frame number. Similarly, a further windowing (W) unit 12 is
provided to convert the frames into a regular digital signal. The
windowing units 11 and 12 may comprise overlap/add circuits for providing
partially overlapping frames.

[0052]The audio system may further comprise one or more amplifiers,
filters and/or signal processing means (not shown). Suitable D/A
(Digital/Analog) converters may be provided for converting the digital
output signal into an analog output signal. Similarly, an A/D
(Analog/Digital) converter may be provided at the input for converting
(that is, digitizing) an analog input signal into a digital input signal.
The audio system 10 may further comprise one or more transducers, such as
loudspeakers, for producing sound. Additionally, or alternatively, the
audio system 10 may comprise one or more sound signal sources, such as a
DVD player, a CD player, MIDI storage, and/or an Internet terminal.

[0053]The present invention is based upon the insight that reverberation
may be generated efficiently using a delayed transformed signal instead
of a delayed time signal. The present invention benefits from further
insight that the delayed transformed signal should have a substantially
random phase.

[0054]It is noted that any terms used in this document should not be
construed so as to limit the scope of the present invention. In
particular, the words "comprise(s)" and "comprising" are not meant to
exclude any elements not specifically stated. Single (circuit) elements
may be substituted with multiple (circuit) elements or with their
equivalents.